In particular, certain biologically active substances, for example drugs or enriched feeds, fed to ruminants can be enzymatically destroyed as they pass through the rumen, the amount of destruction being flavored by the conditions in the rumen, for example residence time (a few hours to several days) and by the pH (from 5 to 6).
It is consequently important to protect these biologically active substances by coatings which are stable at a pH greater than or equal to 5, that is to say which are stable in the rumen of ruminants, which withstand degradation by microorganisms and which permit the release of the biologically active substances in a part of the digestive tract, more especially in the abomasum, in which the pH is less than or equal to 3.5. Whereas the period of protection in the rumen has to be relatively long (several hours to a few days), the release of the active substance in the abomasum must take place within a relatively short time (from a few minutes to 1 or 2 hours).
To obtain such results, it is advantageous to be able to have coatings for the active substances such that they are insoluble in the rumen at a pH of from 5 to 6, but soluble, dispersed or greatly swollen in the abomasum at a pH of less than or equal to 3.5 to release the active substance.
To produce such coatings, it has previously been proposed to use, inter alia, copolymers of maleic anhydride with another monomer, the copolymers being modified by the action of a primary/tertiary diamine on the anhydride groups, thereby forming aminated imide groups which provide the desired solubility, see, for example, French Patent 1,536,774. It has also been proposed to use aminated cellulose derivatives obtained from an unsaturated derivative of cellulose, for example an ether or ester, which is reacted with a nitrogenous compound containing an active hydrogen atom, such as piperidine, morpholine or a secondary amine, see, for example, French Patent 2,081,320.
Furthermore, in British Patent 1,137,214, Australian Patent 45,117, Belgian Patent 885,654, South African Patent Application 70/04,813, French Patent 2,246,572 and U.S. Pat. No. 3,341,505, there are described copolymers of:
a) a neutral ethylenic monomer such as methyl acrylate or methacrylate, styrene, acrylonitrile or vinyl acetate, and
b) a diethylenic monomer containing a basic nitrogenous group such as diethylaminoethyl acrylate or methacrylate, morpholinoethyl acrylate or methacrylate or a vinylpyridine.
To coat feeds intended for ruminants, it has also been proposed to use styrene/vinylpyridine copolymers containing hydrophobic substances chosen from fatty acids containing 10 to 32 carbon atoms and polycarboxylic acids comprising 10 to 22 carbon atoms per carboxyl group which improve the protection by reducing the overall sensitivity of the coating film to weakly acidic aqueous media, see, for example, French Patent 2,401,620. In such coating compositions, the hydrophobic substance enables the wettability of the polymer to be decreased but remains devoid of an effect on the release of the active principle in an acidic medium.
In French Patent 2,514,261, a coating is described which consists of a copolymer sensitive to pH variations, chosen from copolymers of styrene with vinylpyridines, and a water-insoluble polymer insensitive to pH variations, chosen from cellulose acetate butyrate, ethylcellulose and cellulose propionate, the latter promoting the release of the active substance at a pH of from 1 to 2.5 and enabling the extractability of the active substance in an aqueous medium to be reduced.
In French Patent 2,401,621, a hydrophobic polymer in which there is dispersed a substance which is soluble in an acidic medium, for example an alkali metal phosphate, dispersed in crosslinked basic polymers, is described.
In U.S. Defensive Publication number T 100,404, a double-layer system is described, in which the active substance is coated with two successive layers consisting of a polymer sensitive to pH variations, such as a styrene/vinylpyridine copolymer, and a hydrophobic substance such as a fatty acid, in different proportions.
The present invention provides a composition which is stable in a medium in which the pH is greater than or equal to 5.5 and which permits the release of an active substance in a medium in which the pH is less or equal to 3.5, comprising the active substance, which contains inclusions of, or which is completely or partially covered by, a pH sensitive material whose extent and/or rate of swelling is greater in an acidic medium than in a neutral medium, and a coating of a hydrophobic layer.
The pH sensitive materials which are especially suitable are basic polymers, salts of natural or synthetic acids or polyacids, proteins, polysaccharides such as polyglucosamines or alginates, or mixtures of any two or more of these. Examples of basic polymers are those containing at least one basic amino group and having a basic nitrogen content of from 2 to 14%, such as aminated derivatives of cellulose, polymers and copolymers of aminated derivatives of acrylic, methacrylic and crotonic acids, and polymers or copolymers of styrene or acrylonitrile with isomers or derivatives of vinylpyridine, such as 2-vinylpyridine, 4-vinylpyridine or 2-methyl-5-vinylpyridine. Examples of salts of natural or synthetic acids or polyacids are calcium carbonate, zinc polymethacrylate or complex polyphosphates of calcium, sodium, aluminium or magnesium. A preferred protein is zein, which can be isolated from maize gluten. Preferred polyglucosamines are chitosan, which can be obtained by deacetylation of chitin which is found in abundance in the shells of crustaceans, and chitosan derivatives.
The most preferred pH sensitive materials are those polymers which contain at least one basic amino group and which have a basic nitrogen content of from 2 to 14%.
These pH sensitive materials can be used alone or mixed, or in combination with adjuvants.
The hydrophobic layer is preferably chosen such that it possesses a texture which permits the diffusion or penetration of the external liquid medium. In addition to a low permeability to water, the hydrophobic layer must have suitable physical properties, such as high tensile strength and yield strength, and good film-forming qualities.
Examples of hydrophobic substances which make up the hydrophobic layer are fats, paraffin waxes, natural waxes (carnauba wax, beeswax), synthetic waxes (polyethylene wax), polymers such as polyethylene, polypropylene, polybutenes, polyisobutenes, polypentenes, polystyrene, polyvinyl chloride or fluoride, polyvinylidene chloride or fluoride, polyphenylenes, polyphenylene oxides, polybutadiene, polyisoprene or polychloroprene, polyvinyl acetate, water-insoluble cellulose derivatives and latexes; the hydrophobic substances can be used alone or in a mixture, so as to obtain an external layer having the desired mechanical properties.
To obtain a thin coating, the hydrophobic substances should have a suitable viscosity in the molten state or in solution. It is especially advantageous that the viscosity in the molten state is from 20 to 100 poises (2 to 10 Pas). When the viscosity is too low, coating is unsatisfactory. When the viscosity is too high, the coating layer is too thick and agglomeration phenomena arise.
In order to lower the viscosity, it is possible to mix substances which are liquid at room temperature with solid substances. The liquid substances should be compatible with the polymer.
Preferred hydrophobic substances are the readily melted mixtures of polyethylene wax, paraffin and hydrocarbon resins.
In the compositions according to the invention the pH sensitive material preferably represents from 0.5 to 30% of the weight of the active substance and any adjuvants which may be present in association with the active substance. The external coating layer, the average thickness of which is preferably from 5 to 200 micrometers, depending on the size of the granule to be coated, preferably represents from 1 to 50% of the total weight of the composition.
The compositions of the present invention can contain adjuvants to facilitate the preparation of the compositions or to improve their physicochemical characteristics. It may be advantageous to add plasticizers (triacetin, propylene glycol), lubricants (magnesium stearate), binding agents (polyvinylpyrrolidone, polyvinyl alcohol, gelatin), antistatic agents (triglycerides having polyoxyethylenated chains), anti-caking agents (silica, calcium carbonate), fungicides, emulsifiers (oxyethylenated sorbitan esters, sugar glycerides), compatibility agents (natural or semi-natural gums such as alginates, gum tragacanth, pectins, carragheenates, xanthan gum), cellulose ethers (carboxymethyl-, methyl- or hydroxypropylcellulose), inorganic fillers (inorganic salts), sugars, starches or proteins. These adjuvant derivatives generally represent only a few percent by weight of the total weight of the coating.
The active substances can be therapeutic or nutrient substances such as drugs, vitamins (vitamin A, vitamin E) or amino acids, preferably essential amino acids (lysine, methionine) intended for oral administration to ruminants. The therapeutic or nutrient substances are generally in solid form; when the therapeutic or nutrient substances are in liquid form, they may be adsorbed on an inert carrier such as silica, a silicate, an alumina, an aluminate, a silicoaluminate or starch.
The compositions according to the present invention are preferably granules, which are generally spherical or cylindrical, their average diameter being from 0.05 to 5 mm. They can be prepared by application of the known techniques of granulation and coating.
In general, it is necessary to mix the active substance with, or to completely or partially pre-coat it by, the pH sensitive material and then to coat the product thereby obtained, generally in the form of granules, with the hydrophobic layer.
The mixture of the active substance with the pH sensitive material can be made by mixing the finely divided substance and material and then granulating the product obtained.
The partial or complete pre-coating can be performed by the customary coating techniques, such as encapsulation in a fluidized bed, immersion or coacervation.
To perform the coating with the hydrophobic layer various techniques can be used. It is possible to perform the coating, for example, in a fluidized bed, by immersion, by adsorption in a liquid medium or by coacervation. It is also possible to perform the coating with the molten or dissolved hydrophobic substance by projecting a suspension of the pretreated active substance in the molten substance, or the substance dissolved in a suitable organic solvent in which the pretreated active substance is insoluble, onto a flat or concave disc, optionally grooved, rotating at a specified speed and heated by means of hot air, in general to a temperature 20.degree. above the solidification temperature of the hydrophobic composition. In general, the pretreated composition containing the active substance is dispersed in twice its weight of hydrophobic composition. The excess hydrophobic substance forms small particles which remain in the vicinity of the disc, whereas the required coated particles are ejected further away. This results in systematic separation of the required coated particles and the particles of hydrophobic substance in the course of carrying out the process. The excess hydrophobic substance can be recycled.